AERIAL PHOTOGRAPHY: PRINCIPLES

AERIAL
PHOTOGRAPHY:
PRINCIPLES
Frame capture sensors: Photographic and digital
cameras
• Introduction
– Frame vs scanning sensors
• Cameras (film and digital)
• Photogrammetry
• Orthophotos
OVERVIEW
• Air photos are collected using a frame-based sensor, while
most other remote sensing products are obtained using a
scanning or pushbroom sensor (e.g.) (LiDAR uses scanners).
FRAMES VERSUS SCANS
• What is a significant benefit of frame-based sensors over
scanning sensors?
– Hint: geometry
FRAMES VERSUS SCANS
Every aerial mapping camera
superimposes fiducial marks on
each photo. The fiducial marks
can be used to determine the
principal point (+) of the photo, as
well as to determine if the photo
is distorted (compare the measured
distances between the fiducial
marks to the known distances).
AERIAL MAPPING CAMERAS
Example aerial photo specifications Go to page 17 Section 2-02.1
COLOUR FILM CHARACTERISTICS
Panchromatic film
(black and white)
Colour film
FILM SPECTRAL SENSITIVITIES
COLOUR FILM PROCESSING
Blue-absorbing filter is placed in front of the film (e.g., on the lens)
COLOUR IR FILM
CHARACTERISTICS
COLOUR IR FILM PROCESSING
• When working with film we need to be concerned with its
sensitivity - how little light (photons) can be used to activate
the particle and what range of light can be detected. The
sensitivity is related to the speed of the film. The finer the
crystal the 'faster' the film. Larger grains are more sensitive
with a bigger dynamic range, but larger grains result in lower
spatial resolution. Finer grains produce higher resolution, but
with less sensitivity.
Up to the equivalent of
200 megapixels in an
9 × 9” photograph
with fine grain film
FILM SENSITIVITY
10 MB pixel camera
216 MB pixel camera, capturing both
panchromatic and multispectral images
Vexcel Imaging’s Ultramap Platform
DIGITAL CAMERAS
Panchromatic film
IR film
FILM CHARACTERISTICS
Panchromatic film
IR film
FILM CHARACTERISTICS
Natural colour film
IR colour film
FILM CHARACTERISTICS
Natural colour film
IR colour film
FILM CHARACTERISTICS
• Photogrammetry: geometric properties
about objects are determined from
photographic images. Photogrammetry is as
old as modern photography and can be dated
to the mid-nineteenth century.
• Photogrammetry allows for the extraction of
three-dimensional features from remotely
sensed data (close-range, aerial, orbital, etc.).
THE SCIENCE OF MEASURING
PHOTOS
• The majority of aerial photos are taken for photogrammetric
purposes (e.g., to provide information to be used in the
creation of a topographic map).
• To meet those purposes, stereo aerial photographs are
required.
OBTAINING AERIAL PHOTOS
• How is it that we can use overlapping photos to view a
landscape in 3-D?
STEREO VISION
STEREO PHOTOGRAPHY
OBTAINING STEREO COVERAGE
Photogrammetric control surveys
PRODUCING TOPOGRAPHIC MAPS
Very sophisticated devices
used to derive precise coordinate information
Now, most photogrammetric projects
are completed using softcopy
photogrammetric software.
STEREOPLOTTERS
TOPOGRAPHIC MAPS
• Compare the map and photograph below. Both show the
same gas pipeline, which passes through hilly terrain. Note
the deformation of the pipeline route in the photo relative to
the shape of the route on the topographic map. The
deformation in the photo is caused by relief displacement. A
single photo does not serve well as a source for topographic
mapping.
AIR PHOTO ‘DISTORTIONS’
• Relief displacement: the radial distance between where an
object appears in an image versus where it should be
according to a planimetric coordinate system (the datum
plane). Displacement is radially outward for features above
the nadir elevation, and inward for features below the nadir
elevation (on the film plane).
Relief displacement
results in the tops of
the buildings to appear
as if they were leaning
outwards from the centre
of the photo.
AIR PHOTO ‘DISTORTIONS’
• Relief displacement—a simple
explanation.
– Consider point A. On a map (the
‘datum plane’), A would appear at
point A’ (orthographically projected
down from A). A photograph of the
map would show A’ at point a’.
However, in the actual photograph A
shows up at point a”. Point a” is
displaced outward from point a’.
– Similarly, point B would appear at B’
on a map. On the photo point b’ (the
photographed ‘map’ position of B) is
further away from the nadir
(represented as point N / n) than is
point b” (the actual position of B in
the aerial photo--B is displaced
inward).
AIR PHOTO ‘DISTORTIONS’
Trying to fly the plane in a straight, level path is often difficult.
Yaw
Notice how the photos
don’t line up, and how
they are ‘twisted’ relative
to each other,
which indicates that
the flight path was not a
perfect straight line.
AIR PHOTO ‘DISTORTIONS’
• Orthophotos are created by (conceptually) draping the air
photographs over a DEM, carefully mosaicing them together;
removing the ‘distortions’ associated with the perspective
projection, scale differences, etc. (i.e., by ensuring that the
scale is constant everywhere in the photo).
ORTHOPHOTOS
• Four basic operations or corrections must be applied to the
standard vertical aerial photograph to produce an
orthophoto:
– standardization of scale across the image (i.e., use a
DEM to normalized the distance from the camera to the
‘ground’)
– removing the relief displacement to position the terrain
in its true location.
– assignment of ground coordinate values (e.g., UTM
eastings and northings) to the image.
– The final task involves the radiometric or tonal
adjustments to allow the image to blend with
neighboring images. (a complete example)
ORTHOPHOTO CONSTRUCTION
Orthophotos provide the same view of the landscape
(an orthographic one) as do maps.
Aerial photos
provide a
perspective
view of the
Landscape.
Orthophoto
ORTHOPHOTOS
• Orthophotos are metric photos; aerial photos are not metric.
The qualities of aerial photos that enable us to see stereo images mean
that any single aerial photo presents a non-metric view of the landscape.
ORTHOPHOTOS
• A comparison of an aerial photo with an orthophoto.
Note how the linear feature is distorted in the aerial
photo, but is straight (as it should be) in the orthophoto.
ORTHOPHOTOS
• Using stereoplotters (analog or
digital [softcopy]) many ‘layers’
of information can be derived
from aerial photos; as well, if
orthophotos are produced the
resulting images can also be
used in a GIS.
AERIAL PHOTOS -> GIS
• Introduction
– Frame vs scanning sensors
• Cameras (film and digital)
• Photogrammetry
• Orthophotos
SUMMARY